Initial neutronics analysis of long-life cores with small burnup reactivity swing

The neutronic feasibility of designing small modular cores that are simple to operate, have enhanced safety, and long lives due to nearly zero burnup reactivity swing is investigated. The main desired characteristics of such cores include: modular arrangement, low-to-medium power (around 100-300 MWth), good proliferation resistance, simplified control, reduced maintenance, improved safety with emphasis on passive characteristics, minimized and simplified waste disposal. Most of these characteristics are the consequence of the long life of the core-of the order of 15 years without either fuel shuffling or refueling. A long core life can be achieved for various configurations and combinations of fissile fuel, coolant, structural materials and possibly moderator. A couple of approaches are pursued in our studies: (1) designing for high enough conversion ration so that Kinf is maintained nearly constant over the fuel life, and (2) designing for large excess reactivity at BOL. The first design approach calls for a hard neutron spectrum. In order to achieve a hard neutron spectrum, a very small water-to-fuel volume ratio is needed, or a very heavy coolant (such as liquid lead) need to be used. In this paper we will present some of the neutronics characteristics of the following long-life cores that we investigated: (a) Small Pb-cooled and reflected cores that are fueled with either Pu-U-Zr or U-Zr with 10 weight % of Zr, (b) Small water-cooled cores that are fueled with either MOX or UO2. The main constrains in the proposed designs included the following: fuel should contain less that 20% of fissile material due to non-proliferation requirements, the peak fuel burnup should not exceed 150 GWD/tHM, and the maximum neutron fluence on the cladding for neutron energy above 0.1 MeV should not exceed 4 x 1023 n/cm2 .(author)